Low cost WDM systems usually operate on a single fiber transporting traffic in a bi-directional way. These systems support point-to-point connections, where one terminal is located in a central hub for all connections (head-end terminal), while the second terminals (tail end) of each point-to-point connection are locally distributed. The traffic in the head-end to tail-end direction is transported in a first wavelength band, e.g. the optical L-band, while the traffic in the opposite direction is transported in a second wavelength band, e.g. the optical C-band. This reduces signal deterioration due to backscattering or reflections which would be present if identical wavelength bands were used for both transmission directions. Currently, the architecture of such a system has a tree structure, in which all connections use dedicated channel signals forming a WDM signal in one or more trunk fibers and in which the individual channel signals at respective optical carrier wavelengths, i.e. optical channel wavelength bands, are split in a remote optical filter unit. For each point-to-point connection between the remote filter unit and the tail-end terminal only a single wavelength per direction is used per fiber. However, it is also possible to use double or multifold point-to-point connections between the head-end terminal and the same tail-end terminal or to use more than one optical channel signal for a single point-to-point connection between the head-end terminal and a tail-end terminal, if the bandwidth provided by a single optical channel signal is insufficient to transport the desired information.
For some applications it is required to protect the connections against fiber cuts and other external impacts in order to increase the system availability. Using the architecture described above, protection can only be guaranteed by doubling the fiber tree structure, leading to high costs. Protection of two-fiber optical transmission systems using one fiber uni-directionally for each transmission direction is usually achieved by using a fiber ring and transporting traffic in both directions over each fiber. In these systems, the head-end to tail-end traffic uses the same wavelength band as traffic in the other direction. However, using two optical fibers in order to achieve protection increases the costs for realizing such structures.
Further, a protected optical single-fiber transmission system is, for example, disclosed in U.S. Pat. No. 6,327,400 B1. This system uses a ring structure to assure the protection. The tail-end terminals are coupled to the single-fiber ring by means of two 1×2 couplers which enable a communication in both directions of the ring. Here, an optical signal in the form of a TDM signal is used so that an optical signal (the TDM signal transported by a single optical carrier signal) is used for both communication directions. Thus, the bandwidth must be shared by all the tail-end terminals, so this structure is not appropriate for applications that require an increased bandwidth for the communication between each tail-end terminal and the head-end terminal.